1. Field of the Invention
The present invention relates to apparatus for treatment of a liquid in one or a series of substantially horizontal cylindrical elongated tubes. More particularly the invention concerns apparatus in which a cylindrical tube filled with liquid undergoing a process such as heat exchange, mixing, crystallization and precipitation, involving the settling out of solid material from the liquid, is provided with means for agitating the liquid and cleaning the deposits from the interior surface of the tube.
2. Description of the Prior Art
Apparatus, generally referred to as scraped surface heat exchangers, have heretofore been applied for instance in the treatment of petroleum oil for the purpose of dewaxing the oil, in the crystallization of organic chemicals and in heating or cooling viscous materials.
The desired heat transfer in such apparatus is generally brought about by surrounding the abovementioned cylindrical shell with means for sweeping the outer surface of the cylindrical tube with fluid for conveying heat. Such means is commonly provided in the form of an outer concentric cylindrical tube connected to a supply of flowing heat transfer fluid, e.g. chilled brine, and apparatus in this form is referred to as a double pipe heat exchange unit.
The transfer of heat to or from substances having a high viscosity or which crystallize under certain conditions cannot be procured economically in an ordinary heat exchanger. These substances form crystals or adhesive films on the heat transfer surface which considerably impair the rate of heat transfer between the substance and the heating or cooling medium. By means of spring loaded scraper blades constantly removing the films or crystals the rate of heat exchange could be maintained at an economic level.
The means for cleaning deposits from cylindrical shell surface accordingly normally comprises one or more pairs of scraper blades in the form of metal strips mounted to abut, in spring loaded manner against the surface to be scraped, on a rotary drive shaft coaxial with the surface of the cylindrical shell. This central shaft is in turn mounted in bearings at least at one end of the tube, usually at each end, and generally also at intermediate points along its length, and extends outside the tube through a sealing gland to be coupled to drive means for rotation of the shaft, whereby deposits are continually removed which would otherwise accumulate and not only hinder liquid flow but also reduce heat transfer efficiency.
Typical dimensions of such apparatus include a unit pipe length of 40 feet and an internal diameter of 12 inches. It will be apparent that the rotating assembly of scraper blades and shaft must be supported at regular intervals along the shaft to prevent the assembly resting on the lower part of the pipe. For this purpose plain bearings are fitted and the whole bearing lubrication requirements must be met by the liquid being processed. In the case of certain such liquids the lubrication properties are far from ideal. Bearings of a design suitable for all liquids would be difficult to justify on economic grounds because of the generally rough standard of scraped surface heat exchanger manufacturing tolerances.
A typical heat exchange unit of the kind described above conventionally forms part of a double pipe heat exchanger composed of a plurality of such units connected in tiers for the series flow of liquid to be treated therethrough. Even when serially connected in this way, however, each unit usually has to be of great length, and it is often a problem to maintain constant the force with which the blades contact the tube surface. This problem is caused by the relatively reduced support for the weight of the shaft and scrapers in sections remote from the bearings, and leads to incomplete cleaning of the pipe surface above the level of the shaft, with consequent reduction in heat exchange efficiency and obstruction of flow.
These weight and bearing problems increase as the diameter of the equipment is increased.
In U.S. Pat. No. 3,587,729 a double walled tubular heat exchanger of the general type described above is disclosed in which pneumatic or hydraulic adjusting devices are arranged between the blades and the supporting shaft, for pressing the scraper blades against or for lifting them away from the inner wall of the heat exchanger. This apparatus is not, however, addressed to the problem of bearing wear and variations in scraper pressure due to the weight of the shaft between supports, and its use would aggravate rather than alleviate these problems.
In U.S. Pat. No. 3,607,400 a scraped surface double pipe chiller is described with blades carried on and rotated by a central shaft journalled at one end, the cleaning action being realized by longitudinal reciprocation of the rotating shaft, so that the blades, which occupy only a small proportion of the length of the shaft at the end thereof and scrape only a short length of the vessel wall at one time, may reach different parts of the vessel surface. Reference is made to the problem of contacting the entire inner surface of the vessel with equal force, caused by the actuating shaft and scraping equipment at the end of it being heavy and having a density different from that of the liquid within the scraping action is being performed. The problem is approached in this U.S. patent by providing an actuating shaft having an adjustable weight per unit length and which can be constructed so that the weight of the shaft is substantially equal to the weight of the liquid displaced by the shaft.
In U.S. Pat. No. 3,731,339 an attempt is described to extend the application of a buoyant shaft to a scraped surface double pipe heat exchanger having a hollow sealed central shaft supported in bearings at each end at intermediate points and carrying scraper blades along substantially all its length on a superstructure of hollow members sealed against entry of liquid. In spite of the predominance of hollow members the structure only achieves a reduction of 25 percent in the effective weight of the scraper assembly.
The problem of equalizing scraper action in blades mounted along the whole length of a conventional double pipe heat exchanger unit has thus not hitherto been solved.